Radiolabelled compounds which are subject to localization in particular organs or tumors therein are of great value for the diagnosis of diseases of the human body. For example, iodine-123 labelled fatty acids and thallium-201 have been utilized as heart imaging agents. Various isonitrile ligands labelled with technetium-99m have been used to image infarcted regions on the heart.
Substituted benzodiazepines, which are labelled with iodine-123, iodine-125 or iodine-131 have been used for studies of benzodiazepine receptors in the brain.
A new benzodiazepine receptor ligand [radioiodine]3-(5-cyclopropyl-1,2,4-oxadiazol-3-yl)-7-iodo-5,6-dihydro-5-m ethyl-6-oxo-4H-imidazo[1,5-a][1,4]-benzodiazepine was introduced recently (C. Foged et al., J. Nucl Med. 1993: 34, p89).
The common preparation scheme for radioiodined carrier free neuroreceptor ligands employs a trialkyltin precursor and radioactive iodine by the chloramin-T method in strongly acidic conditions. However the labelling reaction is associated with a large and consistent formation of a radioactive volatile reaction product: methyliodine (Y. Zea-Ponce et al., J. Lab. Comp. Rad. 1994: 36, 331-337).
The side reaction products may contain up to 60% of the activity that is added into the reaction mixture. The loss of that amount of the activity increases production costs tremendously.
An another disadvantage, which results from the formation of methyliodine, is radio protection costs and problems. The high increase of the required radioactivity increases the need for radioprotection of persons in the very early moments during upscaling the process. Another radioprotection problem resides in the need to catch up methyliodine. Active charcoal filters, which are used to capture radioiodine from the air, do not hold methyliodine. It requires special reactor grade active charcoal filters that are another source of extra expenses.